[TenTec] Centurion on AM or other carrier modes

Mon Dec 19 22:19:31 EST 2005

Rob:

Glad you pointed out the concerns.  Glad to do it.

73
Bob, K4TAX

----- Original Message ----- 
From: "Rob Atkinson K5UJ" <k5uj at hotmail.com>
To: <RMcGraw at Blomand.Net>
Cc: <tentec at contesting.com>
Sent: Monday, December 19, 2005 8:42 PM
Subject: Re: [TenTec] Centurion on AM or other carrier modes

> Bob,
>
> I thank you for your correction and i think you have  now set the record
> straight.
>
> All the best,
>
> Rob / k5uj
>
>
>>From: Robert & Linda McGraw K4TAX <RMcGraw at Blomand.Net>
>>To: Rob Atkinson K5UJ <k5uj at hotmail.com>
>>CC: Tentec <tentec at contesting.com>
>>Subject: Centurion on AM or other carrier modes
>>Date: Mon, 19 Dec 2005 18:58:08 -0600
>>
>>Rob:
>>
>>First I have no problem in making my comments open to the reflector and I
>>am doing so with this communication.  Originally, I elected a private
>>e-mail due to the original subject matter.
>>
>>As to running the Centurion at the 250 watt power level, it should be
>>understood that my modulation in the AM mode, due to external asymmetrical
>>processing, on positive peaks exceeds 100%.  Thus it would not be correct
>>to run at the 300 or 375 watt carrier level based on a PEP rated output of
>>1.2 KW.  This is my reason for running in the 200 to 250 watt power level.
>>I do monitor my modulation with a scope, SB-610, and depending on
>>processing, I do adjust the drive to attain a carrier level which allows
>>full positive modulation without peak compression regardless of the actual
>>unmodulated carrier value.
>>
>>In fact, 250 watts with 125% positive modulation is more effective in
>>communications than 350 watts at 100% modulation.  And 250 watts lets the
>>amp run a bit cooler as the duty cycle of the carrier is less (21%) as a
>>percentage of rated output.
>>
>>Regarding tuning, here is a quote from my manual:   "11] Many operators
>>prefer to "rag chew" at reduced power levels (500 watts, or so, output) 
>>but
>>want the option to abruptly increase power to 1200 watts without 
>>re-tuning.
>>To do this you must first tune the Centurion for 1200 watts output.  You
>>can then reduce power to the desired level by reducing the drive from your
>>exciter.  The grid current will go down as you reduce drive and that is 
>>OK.
>>The amplifier will not be quite as efficient at reduced power levels using
>>this method but the 3-500Z tubes will not be damaged.  The efficiency of
>>the amplifier at 500 watts can be improved by retuning but remember, if 
>>you
>>then increase drive you MUST re-tune the amplifier."
>>
>>Keep in mind that with AM operation, I am concerned with the PEP value, 
>>and
>>thus not to operate such that peaks are compressed hence the maximum PEP
>>value is 1200 watts.  In using this value, I must then tune the amp for
>>1200 watts PEP output while the unmodulated carrier is only in the 200 to
>>300 watt range.
>>
>>I think we are looking at the operation slightly different.  I agree that
>>plate efficiency can be improved by tuning at a specific power level.
>>Doing this, however, clearly abates the definition of "linear amplifier".
>>In true terms, a linear amplifier should produce a given output for a 
>>given
>>input based on gain of the stage up to the maximum rated output.  i.e.
>>using a 10 dB gain stage, then 1 watt input produces 10 watts output, 10
>>watts input produces 100 watts output, and etc.  Should the amplifier 
>>stage
>>not produce the required output based on input vs. gain then the amplifier
>>is into compression and is no longer linear in power function.
>>
>>Of the two amplifiers that I have, the gain runs about 11 to 12 dB
>>depending on band, with 20 meters having more gain due to the
>>characteristics of the input network.
>>
>>Recent measurements on one of my Centurion amps using a Bird 43P and Bird
>>50.5 ohm load determined the following:
>>
>>Input 4.5 watts {10 watt element}
>>Output 65 watts {100 watt element}
>>Gain = 11.9 dB
>>
>>Input 10 watts {10 watt element}
>>Output 145 watts {250 watt element}
>>Gain = 11.6 dB
>>
>>Input 50 watts {100 watt element}
>>Output 800 watts {1000 watt element}
>>Gain = 12.0 dB
>>
>>Input 100 watts {100 watt element}
>>Output 1500 watts {2500 watt element}
>>Gain = 11.8 db
>>
>>I conclude the slight variations in gain as stated above is due to the FS
>>scale range of the Bird slug used and the accuracy therein.  At the same
>>time I accept the 0.4 dB different between  the power levels and
>>calculations as being normal.  Also, I would point out that this amplifier
>>does produce 1500 watts output which is slightly higher than stated in the
>>manual and the published specifications.  I attribute this to operation on
>>20M, which seems to have a wee bit more gain, and I have a slightly
>>elevated line voltage at the amplifier.
>>
>>I still stand solid on the position that the Centurion can be safely
>>operated at the 250 to 350 watt carrier level on AM, if properly tuned and
>>staying within the duty cycle limits of 50% and 10 minute transmission
>>limit.   I've been running the amp since 1991 with original tubes and fan
>>and only one failure which was caused by a flash over of the plate tuning
>>capacitor when it was being operated on 10 meters.
>>
>>In my case, if it fries, then Tentec gets to sell some parts to K4TAX.
>>
>>73
>>Bob, K4TAX
>>
>>
>>snip
>>
>>
>>>
>>>I enjoy your statements and find them educational and thank you for them.
>>>There is nothing there to disagree with, however I believe in your
>>>original comments you created the false impression that the Centurion may
>>>be operated in AM service with a 375 watt carrier.  You wrote in part:
>>>
>>><<<As to AM it works nice, just adjust the power
>>>in AM mode to 25 to 30 watts output, LSB, and run the mike gain where you
>>>would for SSB.  This power level is just about correct for driving a
>>>Centurion to legal AM limit of 375 watts carrier.  A scope on the output
>>>is
>>>a big help in getting modulation set correctly. >>>
>>>
>>>I disputed you on that, and I notice now that you are stating to me
>>>privately that you normally operate your centurion at 250 to 300 w. cxr 
>>>in
>>>observance of the 1.2 kw pep limit.   Regardless of the technicalities, i
>>>believe you are obliged to make public a revision of your earlier
>>>statement, as to not do so, will mislead potential Centurion purchasers,
>>>and do them a disservice.    With a new Centurion at an eye-popping $2700
>>>I think this is kind of important.
>>>
>>>I would also like to point out that in the Centurion manual it is advised
>>>that the operator tune for maximum output for the given drive, i.e. back
>>>off on the drive for 600 w. and tune for max. output there.  TT gives
>>>tuning for 1200 w. then reducing drive for ragchewing as an option, but
>>>recommends tuning for whatever power level is desired as optimal.   This
>>>could very well be a recent addition to the manual.
>>>
>>>Merry Christmas and 73,
>>>
>>>Rob / k5uj
>>
>>
>
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